Spectrally-resolved Rayleigh scattering to measure velocity, temperature, density, and density fluctuations in high-speed flows

Abstract

A new spectroscopic setup to analyze Rayleigh-scattered light from a high-speed jet was created at the NASA Ames Research Center for simultaneous measurement of velocity, temperature, and density. The point-measurement arrangement uses a narrow-linewidth continuous-wave laser and a stabilized, air-spaced, Fabry–Perot etalon to resolve the Rayleigh–Brillouin (RB) spectrum. Light scattered from a 0.4 mm-long and 0.15 mm-diameter probe volume was collected and 10% of the light was split to measure the scattering intensity via a photo-multiplier tube, which provided a measure of the gas density and density fluctuation spectra. The rest was directly imaged through the Fabry–Perot interferometer to an EMCCD camera. A new software program was developed in the Matlab® platform to model the fringes seen in the camera image. At first, a small part of the incident laser light was analyzed to find the instrument function. When Rayleigh-scattered light was passed, a change in the fringe diameter corresponding to the Doppler shift from the air velocity and a thickening of the fringe corresponding to the thermal broadening were observed. A least-squares fit utilizing Tenti’s S6 model of the RB scattering provided velocity and temperature at the probe volume. A high-speed, clean-air jet, operated in the Mach range of 0 ≤ M ≤ 1.2 provided validation of the technique, and demonstrated applicability in shock-containing flows. The compact, transportable, efficient, and affordable setup was fairly accurate: uncertainties were < 7 m/s in velocity, < 5 K in temperature, and < 1% in density. Spectra of density fluctuations were measured over a frequency range of 50 kHz. Principle of Rayleigh scattering technique, (a) schematic representation; (b) superimposed Fabry–Perot images from the present setup, U = 323 m/s, T = 249 K.